Improve detection of jruby platform
Improve detection of jruby platform

liamchristopher/buildr,apache/buildr,jpcaruana/buildr,stewi2/buildr,liamchristopher/buildr,apache/buildr,apache/buildr,jpcaruana/buildr,liamchristopher/buildr,liamchristopher/buildr,stewi2/buildr,jpcaruana/buildr,stewi2/buildr,apache/buildr,jpcaruana/buildr,stewi2/buildr